1. Field of the Invention
The invention relates to a method of determining quantities characterizing the flow of a liquid in a part of a vascular system, which part may be considered to comsist of an entrance vessel, a reservoir and an exit vessel, the method comprising the steps of arranging part of the vascular system in an apparatus for making subtraction images by means of penetrating radiation, introducing a radiation-absorbing contrast medium into the entrance vessel, making a series of subtraction images while the liquid mixed with the contrast medium flows from the entrance vessel, via the reservoir, to the exit vessel, and deriving the quantities characterizing the flow from the differences in radiographic density TD of successive subtraction images, which are due to the displacement of the contrast medium. The invention also relates to a device for determining quantities characterizing the flow of a liquid in a part of a vascular system, which part may be considered to consist of an entrance vessel, a reservoir and an exit vessel, which device comprises dosing means for introducing contrast medium into the entrance vessel and an apparatus for making a series of subtraction images which represent the displacement of the contrast medium in said part of the vascular system.
2. Description of the Prior Art
A method and a device for making subtraction images by means of penetrating radiation (in this case X-rays) are described, for example in EP-A-0 096 941. The radiation passes through an object to be examined and is converted into a visible image by means of an X-ray image intensifier, i.e. the so-called X-ray image. This image represents the transmitted X-rays. According to the subtraction technique used, a "contrast-filled" X-ray image is subtracted from a contrast-free X-ray image. Before the subtraction is carried out, the logarithm of both X-ray images is taken. Thus, a logarithmic differential image or subtraction image is obtained. The density of each pixel in the subtraction image is proportional to the amount of contrast medium present in an elementary volume of the irradiated object which is projected onto the pixel (in both X-ray images). From the density in principle a variety of other values (for example, the local contrast concentration and the local irradiated thickness of the vessel) can be calculated. This density is referred to as the radiographic density.
A method of the kind set forth is known from Radiology 161 (1986), pp. 323-328. In accordance with the known method, the reservoir is formed by a central part of a blood vessel, a proximal part of which constitutes the entrance vessel and a distal part of which constitutes the exit vessel. When two successive subtraction images having the rank n and n+1 in the series are considered, the total radiographic density in the central part of the blood vessel can be measured in the first image n, and in the second image n+1 a part of the central part which adjoins the distal part can be searched in which the total radiographic density is equal to the value measured in the image n. The length thereof is smaller than the length of the central part and the difference is the distance travelled by the liquid (the blood) during the period of time .DELTA.t elapsing between the making of the two X-ray images n and n+1 wherefrom the subtraction images n and n+1 have been formed. When the contrast medium has already progressed as far as the distal part in the first image n, a correction must be made for the amount of contrast material, or radiographic density, which has flowed from the central part into the distal part during the period .DELTA.t. The publication discloses how this correction can be determined.
The known method is of limited use only, because the reservoir must consist of a part of a single, non-branched blood vessel. The measurement of the flow from and to an organ in which the reservoir is formed by branching blood vessels which communicate on the one side with a single entrance vessel and on the other side with a single exit vessel, is not possible. A second drawback of the known method consists in that it can be used only if the concentration of the contrast material in the reservoir exhibits a gradient.